Whatever it is, the actress Anne Hathaway has been subsisting on two portions a day of this joyless-sounding stuff in order to lose 25 lb for the forthcoming film adaptation of Les Misérables, in which she plays the consumptive prostitute Fantine.

“The idea was to look near death,” explained Hathaway, in an interview with American Vogue. “I was in such a state of deprivation, physical and emotional… It took me weeks until I felt like myself again.”

Hathaway is the latest of a growing number of actors and actresses to take up the Ampas Diet: either gain or shed a dramatic amount of weight for a film role, and in a few months the Academy of Motion Picture Arts and Sciences will hopefully reward you with an Oscar.

The results, as many a weight-loss plan has trumpeted, speak for themselves. Jessica Chastain gained a stone to play the curvaceous Celia Foote in The Help, and was nominated for Best Supporting Actress earlier this year. Natalie Portman lost one and a half stone to play a ballerina in Black Swan, and won Best Actress in 2011.

Rewind to 1993 and Tom Hanks lost 26 lb for Philadelphia, before going on to win Best Actor. Seven years later, he dropped 50lb for Cast Away, and secured a nomination. (He lost out to Russell Crowe, who had dropped a mere 35 lb for Gladiator.) In 2004, Christian Bale wasted away to a spectral 8st 5lb for The Machinist, before almost doubling his bodyweight in six months to play the muscular Bruce Wayne in Batman Begins.

And let us not forget the actresses who have reaped the rewards of deglamorisation. Charlize Theron in Monster, Nicole Kidman in The Hours, Renée Zellweger in Bridget Jones’s Diary and Salma Hayek in Frida all temporarily forsook their film-star looks and figures, and picked up two Best Actress Oscars and two nominations respectively.

Of course, there are actors and actresses who regularly delight cinema-goers and critics without having to balloon or deflate into the bargain. So why do the slimmers and the bloaters win so much attention? Well, great acting is a very hard thing to quantify, but few things are more tangible than bony shoulders or a fat bottom. With that obvious proof, coupled with details of the regimen that achieved it in an interview with a glossy magazine, a performer’s commitment suddenly becomes very easy to appreciate.

Here is Hanks talking about Cast Away: “The idea of looking at four months of constant vigilance as far as what I ate, as well as two hours a day in the gym doing nothing but a monotonous kind of work-out – that was formidable. You have to power yourself through it almost by some sort of meditation trickery. It’s not glamorous.”

But it is undeniably impressive – particularly in the age of digital cinema, when actors are frequently slimmed down or fattened up on a computer. Like the protagonist of Kafka’s Hunger Artist, performers who change weight or become ugly for a role want to impress us with their old-fashioned, above-and-beyond diligence.

What makes this craze even odder is that for the first 95 years of cinema’s existence, audiences would have hated it. The screen icons of the past carried their personae from film to film: Charlie Chaplin was almost always the elegantly dishevelled Little Tramp, John Wayne the gruff alpha gunslinger, Marilyn Monroe the pillowy goddess with a voice like toast and honey.

To find out what changed, we must go back to the original Oscar-winning Hollywood yo-yo diet. Robert De Niro rebuilt himself twice for Raging Bull, his 1980 film with Martin Scorsese: once as the young Jake LaMotta, a rippling 11-and-a-half stone prizefighter, and again as his washed-up, 16-stone older self.

De Niro trained with the real LaMotta for almost a year before filming began, fighting hundreds of bouts with the middleweight boxer at the Gramercy club in downtown Manhattan. LaMotta entered him anonymously into three professional bouts – two of which he won. Once the early scenes had been filmed, then came the weight gain, which De Niro achieved by eating his way around Europe for four months while filming was suspended.

When Raging Bull was released, the media obsessed over De Niro’s physical transformation, which irked the film’s co-writer Paul Schrader. “Dieting is not acting,” he said later, “but De Niro was the first one to do that kind of thing. It would’ve been a gimmick if he wasn’t so good… but I think it’s the gimmick that won him the award.”

Today, Raging Bull is rightly regarded as a classic, but at the time Scorsese thought it would be the last film he would make. In the preceding five years, the wild success of Star Wars had changed the business beyond recognition. Tom Shone, author of Blockbuster: How Hollywood Learnt to Stop Worrying and Love the Summer, pinpoints Raging Bull as the moment serious cinema learnt how to punch back.

“In many ways, Raging Bull feels like Scorsese and De Niro’s response to Lucas’s space epic – an anti-blockbuster built to resist the gravitational pull of the Death Star by means of a spectacle no less visceral or intense,” he wrote. “You give us exploding planets, we give you a ballooning Robert De Niro.”

The same fight continues today. Would we be quite as intrigued by Les Misérables if its director, Tom Hooper, cast a skeletal unknown as Fantine instead of Hathaway? Well, of course not. Our foreknowledge of that behind-the-scenes transformation is a crucial part of how we will watch the film: just as crucial, perhaps, as what we actually see on screen.

And as long as we keep eating it up, so Hollywood will continue to diet – and binge, and bloat and uglify – in the name of its art. The oatmeal-paste business is safe for a long time yet.